1. Field of the Invention
The present invention relates to an air pressure type carrier and polishing apparatus provided with a soft retainer ring.
2. Description of the Related Art
In general, polishing a wafer by front side reference gives a higher polishing accuracy than polishing by back side reference, so most carriers adopt an air pressure type structure.
FIG. 13 is a sectional view of an example of an air pressure type carrier of the related art.
A carrier 100 is formed with a depression 101 at its bottom surface. This depression 101 is covered with a sheet 102 to define a pressure chamber R. An air feed path 103 communicating with the pressure chamber R is provided at the center of the carrier 100. A retainer ring 104 for holding the wafer W is attached at the outer periphery of the bottom surface of the carrier 100 by an adhesive (not shown).
Due to this configuration, the carrier 100 can be made to rotate in a state with a wafer W brought into contact with the top of a polishing pad 201 of the rotating lower platen 200. At this time, air is fed from the air feed path 103 to the inside of the pressure chamber R to apply a uniform pressure to the back surface of the wafer W and polish the front surface of the wafer W flat by the polishing pad 201 of the lower platen 200.
In the carrier of the above related art, however, there were the following problems.
Since the retainer ring 104 was in contact with the rotating polishing pad 201 during the polishing of the wafer W, it became worn. In particular, the retainer ring 104 of the related art was set to a thickness of a value in the range from 0.5 mm to 0.7 mm in accordance with the thickness of 6-inch diameter wafers W and 8-inch diameter wafers W. Further, the retainer ring 104 was formed by epoxy glass (EG) or another similarly soft material, so it became worn in a short time, immediately became thinner, and easily peeled off from the carrier 100. Further, when the retainer ring 104 became worn, the wafer W moved upward by the amount of wear of the retainer ring 104 due to the force pressing the wafer against the carrier 100. As a result, the tension caused at the outer periphery of the sheet 102 caused the polishing rate of the outer periphery of the wafer W to become higher than the polishing rate of the center portion, i.e., "face drooping". The degree of face drooping became higher in relation to the amount of wear of the retainer ring 104. From the viewpoint of the allowable degree of face drooping and prevention of peeling, the amount of wear allowed for a retainer ring 104 with a thickness set to a value in the range from 0.5 mm to 0.7 mm is up to about 0.1 mm. Therefore, in a carrier 100 of the related art having a retainer ring 104 set to such a thickness and formed by a soft material such as EG, the number of polishable wafers W produced within the desired range was about 200 sheets, which was extremely poor in terms of mass production.
As opposed to this, it may be considered to form the retainer ring 104 by a ceramic, titanium, or other hard material. While a retainer ring 104 made of a hard material, however, is superior in wear resistance, it is liable to be damaged during polishing work by the outer edge of the wafer W striking the inner peripheral surface of the retainer ring 104.